1. Field of the Invention
The instant invention relates to a process for oxidizing olefins to their corresponding epoxides by contacting said olefin with oxygen in the presence of a multimetallic condition at oxidation conditions. Preferably, this process is utilized to oxidize ethylene to ethylene oxide by contacting ethylene in the presence of oxygen with a silver catalyst promoted by one or more metals combined with silver in an alloy. The promoter metals are selected from the group consisting of platinum, rhenium, palladium, ruthenium, iron, copper, and gold. Preferably, the catalysts used in the process of the instant invention are bimetallic catalysts comprising silver in combination with a promoter metal selected from the group consisting of palladium, ruthenium or rhenium, preferably rhenium and ruthenium, and most preferably rhenium.
The catalysts are prepared by contacting a silver salt or silver oxide having a surface area of from 20-40 M.sup.2 /gm with a solution containing a decomposable salt of said promoter metal, at impregnating conditions, drying said impregnated salt and activating said dried impregnated silver oxide or silver salt by contacting with a reducing environment, preferably hydrogen at a temperature of from 100.degree. to 500.degree.C, more preferably from 150.degree. to 300.degree.C. Catalysts prepared in this manner are characterized as comprising a bulk phase of substantially pure silver and a surface alloy phase of silver and said promoter metal.
2. Description of the Prior Art
The use of ethylene oxidation catalysts comprising silver promoted with various other metals has been taught in the prior art. For example, U.S. Pat. No. 2,605,239 teaches a silver-BeO catalyst which is prepared by a coprecipitation technique.
U.S. Pat. No. 3,144,416 teaches a silver catalyst for use in an ethylene oxide process wherein catalytic silver powder is dispersed in electrolytic silver and molded. Promoters which are useful in forming the catalyst described in this patent include copper, gold, or the oxides thereof.
U.S. Pat. No. 2,143,371 teaches a method for preparing a silver catalyst for ethylene oxidation, which comprises mechanically converting bulk silver, which may be alloyed with gold, copper or iron, into catalytically active flakes. The catalysts produced by this method would have the promoter metal uniformly distributed throughout the catalyst.
German Pat. No. 717,201 teaches that an ethylene oxidation catalyst may be prepared by the reaction of silver nitrate, copper nitrate and gold chloride with hydrazine. This catalyst preparation technique relies on a coprecipitation of all three of the metals from solution, thus, once again, providing a catalyst which is substantially homogeneous with regard to metal promoter concentration throughout the catalyst.
German Pat. No. 1,048,898 teaches preparation of a silver catalyst for ethylene oxide production wherein said silver is in combination with an alkaline earth oxide and a heavy metal promoter. This catalyst preparation technique calls for the coprecipitation of the silver and the heavy metal by use of the alkaline earth material and the deposition of the wet precipitated mass on an inert carrier. This catalyst preparation technique also relies, on coprecipitation and further since an inert carrier is utilized, to support the catalyst, it would be expected that insufficient heat dissipation during use, would be obtained, unlike catalyst of the instant invention wherein silver metal provides the bulk of the catalyst.
German Pat. No. 1,068,684 teaches that a silver catalyst for ethylene oxide production may be prepared by treating a silver nitrate solution containing a few % alkaline earth nitrate with ammonium oxalate and thermally decomposing the resulting silver oxalate, followed by washing, drying and grinding. Once again, this reference teaches the incorporation of promoters, such as small amounts of gold, copper and iron, or peroxides, oxides or hydroxides of barium, strontium or lithium. The catalyst preparation technique disclosed relies on precipitation of a soluble silver salt.
U.S. Pat. No. 2,605,239 teaches the use of silver catalysts wherein beryllium oxide is incorporated as a promoter. The patentee also discusses the use of other promoters, such as copper, aluminum, manganese, cobalt, iron, magnesium, gold, thorium, nickel, cadmium, cerium and zinc. This patent does not recognize the use of platinum, palladium, ruthenium or rhenium as promoters. Furthermore, the patentee incorporates these promoter metals by mechanical mixture or coprecipitation. Thus, it is clear that the patentee does not form multimetallic catalysts similar to the specific alloy-type catalyst of the instant invention.
U.S. Pat. No. 3,664,970 teaches impregnating a silver salt solution into the pores of an alumina support and drying to prepare a supported silver catalyst for ethylene oxidation. The reference teaches that promoter metals can be co-impregnated along with the silver salt. Said promoter metals include barium, copper, mercury and tin. The patentee nowhere recognizes the advantage of utilizing platinum, palladium, ruthenium or rhenium as promoter metals nor does he recognize the advantages of using the unsupported alloy catalysts of the instant invention.
U.S. Pat. No. 2,424,083 teaches the preparation of an ethylene oxidation catalyst by precipitating silver on an inert support by the reduction of an aqueous silver ammonium complex in the presence of said support. This reference teaches that a thin film of silver in the form of a catalytically active silver mirror is deposited upon said support. The patentee discusses the use of promoter metals, such as gold, copper, platinum, nickel and iron, however, there is no discussion of how to incorporate these promoters so as to obtain an alloy catalyst. Furthermore, the patentee does not recognize the advantage of utilizing rhenium, palladium nor ruthenium as a promoter for a silver ethylene oxide catalyst.
Thus, it is clear that none of the references recognize the advantages of using rhenium, ruthenium or palladium as a promoter metal for a silver-based olefin oxidation catalyst nor do these references recognize the use of these metals in combination with silver in alloy form. Furthermore, none of these techniques recognize the extremely improved catalysts prepared by this specific methods utilized in preparing the catalysts of the instant invention, i.e. wherein the catalyst is characterized as having the promoter metal in substantially high concentration at the surface and the bulk portion of the catalyst is substantially pure silver metal.
This distribution of promoter metal allows the most economical utilization of specific promoters which are significantly more expensive than silver while obtaining results equal to or better than the prior art promoted silver catalysts. For example, ruthenium, platinum, palladium or rhenium will be in substantial concentration at the catalytic surface thus allowing much decreased amounts of promoter to be utilized without loss of catalytic effect. Provision for substantially pure silver in the interior of the catalyst may provide more effective heat dissipation than is the case with a catalyst having promoter metals distributed evenly throughout.
Further, none of the instant references appear to teach that a water insoluble silver salt or silver oxide having surface area of from between 20 and 40 m.sup.2 /g is advantageously utilized in forming the silver catalysts useful in the instant invention. It has been found that silver salts or silver oxide with a surface area of less than 20 m.sup.2 /g forms a catalyst which requires high temperatures to achieve reasonable conversions, thus causing decreased selectivity especially in ethylene oxide processes. Silver salts or silver oxide having a surface area of greater than 40 m.sup.2 /gm provides a catalyst which yields excellent conversions but low selectivity to the desired epoxide product. Finally, the processes taught in the prior art which utilize multimetallic catalysts for olefin or ethylene oxidation do not use catalysts prepared by the impregnation technique described below wherein substantially all of the solution of the decomposable salt is impregnated into the pores of the water insoluble silver salt. This technique provides superior multimetallic catalysts since increased contacting of the promoter metal with the silver metal at the surface and the avoidance of agglomeration of separate promoter metal particles upon activation is achieved.